Foreclosure is a lawsuit in which the mortgage bank has the house sold in order to get money to pay back what the homeowner borrowed.

Here is a more official and legal definition of foreclosure:

Foreclosure is the legal and professional proceeding in which a mortgagee or other lien holder, usually a lender, obtains a court ordered termination of a mortgagor's equitable right of redemption. Usually a lender obtains a security interest from a borrower who mortgages or pledges an asset like a house to secure the loan.
[Source: www.wikipedia.org/]

As soon as you stop paying your monthly mortgage payment, the foreclosure "time-line" begins to run.